i don't have springs and i don't drive a Sport so i wouldn't be the best person to advise but there's loads of info out there. have a search and i'm sure you'll find the answers to your questions. failing that Damian over at DPM Performance is a wkd chap and will give you all the advice you want and need.
Depends how low you want to go, H&R are excellent spring but unsure how the dampers on a sport cope with them! S-line springs would be a straight swap as far as i know as loads of people on here have done it!
the trim is different yes but the s-line also has lower springs and different dampers too to the sport , my opinion kirsty is get some h&r or similar springs lots have them on here and highly recomend them failing that the other way is coilovers then you can lower it what you like but quality ones are expensive